1996 S420...This is an interesting problem...the dash button for the trunk release is not working properly...nor is the command from the Keyless entry. When I press the trunk release button or the keyless entry for that matter, I do hear some mechanical noise at the rear of the car...but the trunk does not open. Manually (push the trunk release button in the rear) everything works as advertised.
One other thing...near the left tail light there is a sharp hissing noise (almost like a fart)...I assume that there is a leak in the vacuum line since the pump is actually near the gas tank.
The 4 doors close properly, the doors lock correctly, and the dash button (as well as keyless entry) related to the door locks work correctly...and as I indicated above, the trunk closure mechanism works properly...only the release button on the dash and the keyless entry command as it relates to the trunk.
I love this car...it has some hardware that is truly unique. What a piece of work. To all who comment...Thanks for your input. It will give me something to do over the long weekend.
CK

The remote trunk release is operated by the central locking pump. You need to remove the plastic cover above the trunk lock & find that "hissing" noise when you operate the remote release as that is probably a "pressure" leak in the system causing the problem.

On my 1997 E320, I have the same problem and I removed the trunk lining and I discovered that a vaccum nipple actually broke and I haven't yet fixed it because I don't have time. I tried using tubing to fix the problem but it still leaks. I attached a pic of what I think is broken. On the W210, the part is on the lid. On the W140, I think it is on the trunk deck, so accesibility is harder. I don't think you have to check your vaccum pump because it is easy to hear the hissing from the trunk tumbler.

I haven't come across that part...but I don't think it will be hard to identify thanks to your picture...The noise is coming from right around the left taillight...so it would definately be in the trunk instead of the lid.
I'll let you know what I find.
Thanks again...
CK

The problem wound up being an adjustment issue. The linkage between the vacuum device and the trunk lock mechanism was screwed up. No problem anymore. As for the strange hissing noise, there is an open vacuum line next to the rear brake light on the drivers side. It has a nipple on it...perhaps it is supposed to be a vent line???...it is red in color whereas the rest of the lines in the trunk are yellow. There is probably a fitting of some sort missing to muffle the sound...Any ideas let me know...
Thanks for the help...
CK